In the developing rat brain the choline acetyltransferase and dopamine stimulated adenylate cyclase activity are extremely low at birth and reach adult values 14 to 28 days after birth. Acetylcholine concentrations are 25% of adult values at birth, increase significantly between 7 and 14 days, and reach adult values at about 28 days, while cyclic AMP levels are highest at birth and decrease thereafter. These results support the view that a functional dopaminergic system precedes a functional cholinergic system.